1. Field of the Invention
The present invention relates to an apparatus for controlling an induction motor by using a slip frequency control type vector control system and, more particularly, to a three-phase induction motor control apparatus which is enabled to contral a three-phase induction motor highly precisely with a simple circuitry.
2. Description of the Prior Art
Generally speaking, the induction motor is widely used as a prime mover aiming mainly at a constant run partly because it has the simplest construction among various types of electric motors and partly because it is small and strong, can cause a commercial a.c. power source and can be produced at a reasonable cost. In case a precise variable-speed control is demanded, on the other hand, a d.c. motor is ordinarily used. This is because the d.c. motor can freely control its output such as torque or velocity in accordance with the characteristics required for a load by controlling the current or voltage of an armature or the current of a field system.
Despite of this fact, however, the d.c. motor is equipped with mechanically wearing parts such as a commutator, brushes, etc., so that it has to be subjected to periodic maintenance, inspection, and replacement of parts. Moreover, the d.c. motor cannot be used in an atmosphere of explosive or corrosive gases because it has the aforementioned mechanically wearing parts and has a limitation to a high-speed rotation and a high-voltage supply because its rectification is limited. From its structural requirement, still moreover, the d.c. motor has to be complicated and expensive. Thus, the d.c. motor cannot always be appropriate for a control motor.
These problems are eliminated from the induction motor, which in turn has its control system complicated remarkably in case it is used as the control motor. More specifically, the induction motor finds it difficult to be simply controlled, as compared with the d.c. motor, partly because not only the intensity of its magnetic field but also its torque is influenced if the armature current is controlled and partly because the intensity of the magnetic field is influenced if its frequency control is conducted. Moreover, the control range in this case is far more limited than in the case of the d.c. motor.
As an induction motor control system eliminating this point, there has been proposed a magnetic-field oriented vector control system which is represented by Japanese Patent Publication No. 34725/1975. This system has an effect that a control similar to that of the d.c. motor can be attained while taking the advantage of the induction motor. For practical uses, however, the induction motor has to be equipped therein with a sensor for detecting the magnetic field. This equipment deteriorates the effect of the induction motor, i.e., the simple construction and the freedom from any maintenance and raises the production cost. The control system is complicated because it also has to be equipped with a magnetic flux control unit.
As an induction motor control system having succeeded in improving this point, there have been proposed a variety of other control systems resorting to the slip frequency control type vector control system, as are exemplified by Japanese Patent Publications Nos. 40750/1982 and 32873/1981. In either of these control systems, a current command value is introduced from the difference between a set velocity value and a detected velocity value, this current command value is multiplied by the sine output of a two-phase sine wave generator to determine an active current, a cosine output having a phase difference of 90.degree. from said active component of current is created from the output of said two-phase sine wave generator, this cosine output is multiplied by a preset value to determine a reactive component of current, and the value of orthogonal vector sum of the reactive component of current and said active component of current is used as the control current of each phase.
According to this control system, an excellent vector control can be attained without any requirement for a magnetic flux detector, a magnetic flux control circuit and so on. This control system is appropriate in the case of the two-phase induction motor because it has to introduce reactive component of current having a phase difference of 90.degree. from the active component of current. In the case of the three-phase induction motor, however, the control system has to create a sine wave and a cosine wave for each phase so that its circuitry become complex and difficult and its numerical calculations become troublesome and difficult upon its design or design changes. In case the three-phase induction motor is to be controlled by this system, moreover, a two-three phase converter has to be added for the same reason, which in turn is a factor of complicating the circuitry.